1. Field of the Invention
This invention relates to the detection of infrared (IR) radiation, and more particularly to infrared sensor arrays in which readouts are obtained serially from individual pixel sensors within the array.
2. Description of the Related Art
Pyroelectric focal plane arrays detect patterns of incident IR radiation with an array of pyroelectric elements that produce an electrical charge when heated. The charge generated by each pixel corresponds to the amount it is heated by the incident IR radiation. The individual pixel charges are converted to voltage signals at a remote location, and are then read out as an indication of the IR radiation pattern at the array.
A conventional system is illustrated in FIG. 1, in which an IR focal plane array 2 is alternately shaded and exposed to IR radiation by a rotating chopper blade 4 that has a pair of blade sections occupying opposed quadrants. An AC chopper is used because a DC voltage will tend to decay. The individual pyroelectric pixel elements 6 begin to heat up as soon as the blade has passed by and they are exposed to the IR radiation. Since the chopper blade is considerably larger than the detector array, all of the detector elements 6 are effectively exposed approximately simultaneously. The voltage signals generated from each pixel are read out serially while the array remains exposed. The readout is timed to commence just after the exposure begins, and continues until shortly before the next blade segment arrives to shade the array. The signals from each of the pixel elements are transmitted over a bus line 8 to an amplifying circuit 10 that converts the charge signals to voltage signals and amplifies them to provide an electrical replica of the IR scene.
When the array is shaded by continued rotation of the blade 4, the pixel voltages are reset and negative voltages accumulate as the pyroelectric pixel elements cool off. The array is again read out serially during the shaded phase, at the end of which the pixel voltages are again reset to zero in preparation for the commencement of another exposure cycle.
A drawback of the existing systems is that, since the pixels are readout serially, the first pixel is read when it has just begun to charge up, while the last pixel is read near its maximum charge. One approach towards resolving this problem has been to attempt to read out the pixel signals faster, and then normalize the remaining pixel-to-pixel signal discrepancies that result from the serial readout over time. However, there is a limit to the readout speed that can be obtained, especially for large arrays. 128.times.128 pixel arrays are typical, with the size of each pixel on the order of 50 microns .times. 50 microns. A noticeable time differential is required to read out each of these pixels serially.
Another approach towards compensating for the time delay between reading the first and last pixels has been to read the pixels in one direction while the chopper is open, and then read them in the reverse order while the chopper is closed. While this approach is interesting conceptually, it is undesirable to design multiplexers capable of reading out arrays in opposite directions. Much more complex designs are required, especially for larger arrays.
The chopper blade has also been modified in an attempt to compensate for the readout. As illustrated in FIG. 2, a spiral chopper blade 12 has been substituted for the quadrant chopper blade 4 of FIG. 1. The edge of the chopper blade 12 is configured to traverse substantially an entire row of pixels in the focal plane array 14 simultaneously, but to leave a time increment between the traversal of successive rows. Since the commencement of heating for each row is delayed somewhat compared to the immediately preceding row, the readout delays resulting from the serial readout can be somewhat mitigated. While this approach reduces the location sensitivity of the readout, it requires a large area and relatively expensive chopper blade, and also a very high phase accuracy.
A limitation of both of the chopping schemes described above is that the inherently low level native pyroelectric signal can become overwhelmed by the downstream electronic noise, seriously reducing the output signal-to-noise ratio and the system performance. The small amounts of charge produced by the pyroelectric detectors are distributed across relatively large bus line capacitances between the detector array and the amplifier circuit, and this imposes a further severe limitation upon the generated voltage.